1. Field of the Invention
This invention relates to a fuel cell with increased conductivity.
2. Description of the Related Art
In recent years, various solid oxide fuel cells (SOFC""s, hereinafter referred to as xe2x80x9cfuel cellsxe2x80x9d) have been proposed as next-generation energy sources.
FIG. 2 shows the outline of the structure of a tubular type fuel cell. As shown in FIG. 2, a tubular type fuel cell 10 is constituted by forming a fuel electrode 12, a solid electrolyte (hereinafter referred to as xe2x80x9can electrolytexe2x80x9d)13, and an air electrode 14 as films on a porous cylindrical substrate tube 11 of calcia-stabilized zirconia (CSZ)in this order by the sintering process, and then connecting the fuel electrode 12 and the air electrode 14 together in series by an interconnector 15 as an electrically conductive connecting material. In this tubular type fuel cell, a fuel gas (H2) flows in an axial direction inside the substrate tube 11, while air (oxygen: O2) flows axially along the outside of the substrate tube 11. As the fuel electrode 12, a thermit of Ni and yttria-stabilized zirconia (YSZ), for example, is used. As the air electrode 14, an LaCaMnO3 material, an LaSrMnO3 material, an LaSrCaMnO3 material, an LaSrMnCoO3 material, or an LaSrMnCrO3 material, for example, is used. As the interconnector 15, an LaCrO3 material, an LaMgCrO3 material, or a heat resistant alloy, for example, is used.
FIG. 3 shows the outline of the structure of a planar type fuel cell. As shown in FIG. 3, a planar type fuel cell 20 is prepared by forming a fuel electrode 12 and an air electrode 14 as films on both surfaces of an electrolyte 13 by the sintering process to constitute a cell plate (unit cell) 21, then electrically conductive interconnectors 15 are laid on both surfaces of the cell plate 21 to form a composite, and then binding a plurality of the composites together. In the interconnector 15, a plurality of groove-like fuel channels 22 and a plurality of groove-like air channels 23 for supplying fluids such as a fuel gas (H2)and air (oxygen: O2) are formed so that the fuel gas and air will flow perpendicularly to each other in the directions of the grooves.
The air electrode 14 joined to one side surface of the electrolyte 13 has high reactivity. When the air electrode 14 is integrally sintered at a high temperature of 1,350xc2x0 C. or higher, it may shrink and develop cracks, or may react with the electrolyte 13. The resulting fuel cell may be unable to perform fully. Under these circumstances, it is difficult to sinter the fuel electrode 12 and the air electrode 14 integrally on both surfaces of the electrolyte 13. Thus, the fuel electrode 12 and the electrolyte 13 are integrally sintered at a high temperature of 1,350xc2x0 C. or higher as the first sintering step. Then, the electrolyte 13 and the air electrode 14 are sintered at a temperature of 1,350xc2x0 C. or lower as the second sintering step.
As the properties of the air electrode, the air electrode should increase conductivity, and should be dense in texture, but needs to have some porosity because of the need to diffuse gases. To increase conductivity, a large thickness has been proposed. However, a thick air electrode undergoes cracking or peeling because of its shrinkage percentage. Hence, it is necessary to decrease the shrinkage percentage while maintaining the conductivity, but, prior to the present invention, no satisfactory air electrode has been found.
In light of the foregoing problems, the present invention aims to provide a fuel cell in which an air electrode minimally shrinks when provided in a film form on a side surface of an electrolyte by the sintering process, and which has high electrical efficiency as well as high conductivity properties.
A first aspect of the invention is a fuel cell comprising a fuel electrode and an air electrode disposed on side surfaces of an electrolyte film, the fuel electrode being supplied with a fuel gas and the air electrode being supplied with air, wherein the air electrode is formed from a material selected from the group consisting of LaCaMnO3 materials, LaSrMnO3 materials, LaSrCaMnO3 materials, LaSrMnCoO3 materials, and LaSrMnCrO3 materials, and a particle size ratio, between the average particle sizes of coarse particles and fine particles of the same material or different materials among the materials for the air electrode, is from 5/1 to 250/1. According to this aspect, an air electrode having a decreased shrinkage during sintering and increased conductivity can be obtained. As a result, a fuel cell having high electrical efficiency as well as high conductivity properties can be obtained.
The average particle size of the coarse particles may be from 5 to 500 xcexcm, while the average particle size of the fine particles may be from 0.5 to 2 xcexcm. According to this constitution, an air electrode having a decreased shrinkage during sintering and increased conductivity can be obtained.
The volume ratio between the coarse particles and the fine particles may be from 1/1 to 9/1. According to this constitution, an air electrode having a decreased shrinkage during sintering and increased conductivity can be obtained.
The shrinkage of the air electrode during sintering may be from 0.1 to 5% linearly. According to this constitution, no more cracking or peeling of the air electrode occurs.
The conductivity of the air electrode after sintering may be 50 S/cm or more. This air electrode is preferred for use in a fuel cell.
The porosity of the air electrode after sintering may be from 30 to 50%. Thus, a fuel cell with satisfactory permeability without a decrease in conductivity can be obtained.
The fuel cell may be a tubular type fuel cell or a planar type fuel cell. This fuel cell has high electrical efficiency as well as high conductivity properties.
A second aspect of the invention is a material for an air electrode sintered onto a side surface of an electrolyte in a planar type or tubular type fuel cell, wherein the material is selected from the group consisting of LaCaMnO3 materials, LaSrMnO3 materials, LaSrCaMnO3 materials, LaSrMnCoO3 materials, and LaSrMnCrO3 materials, and the material comprises a combination of coarse particles having an average particle size of from 5 to 500 xcexcm and fine particles having an average particle size of from 0.5 to 2 xcexcm, the coarse particles and the fine particles comprising the same material or different materials among the materials for the air electrode, and the volume ratio between the coarse particles and the fine particles being from 1/1 to 9/1. According to this aspect, an air electrode having a decreased shrinkage during sintering and increased conductivity can be obtained.
A third aspect of the invention is a method for producing a planar type or tubular type fuel cell having an air electrode sintered onto a side surface of an electrolyte in the planar type or tubular type fuel cell, comprising a first sintering step of sintering a fuel electrode integrally on a side surface of the electrolyte at a temperature of from 1,300 to 1,500xc2x0 C., and a second sintering step of sintering the air electrode integrally on the other side surface of the electrolyte at a temperature of from 1,200 to 1,350xc2x0 C., the air electrode comprising the material of the second aspect of the invention, and wherein thermal shrinkage of the air electrode is suppressed during the second sintering step. According to this constitution, an air electrode having markedly decreased shrinkage during sintering and increased conductivity can be obtained. Furthermore, no more cracking or peeling of the air electrode occurs. Consequently, the electrical efficiency of the fuel cell can be maintained stably for a long period.